1. Field of the Invention
The present invention relates to a wire bonding apparatus for electrically connecting a semiconductor chip (hereinafter called "chip") to a substrate with metal wires which are widely used in manufacturing processes of electronic components. Particularly, the invention relates to a wire bonding method and apparatus which prevent the chip from being damaged and which complete wire bonding in a relatively shorter period of time.
2. Description of the Prior Art
FIG. 15 shows the structure of the conventional wire bonding apparatus which includes a bearing 2 fixed on a frame 1 which supports shaft 3 of swing unit 4. A bonding arm 5 is fixed on swing unit 4 which swings along the arrow mark N. Also provided is a capillary tool 7 which is held on the tip of the bonding arm 5. A wire 6 passes through the capillary tool 7 and has on its tip a ball 6a formed by an electric spark. A voice coil motor 8 drives the bonding arm 5 and a ultrasonic vibrator 9 transmits an ultrasonic vibration to the bonding arm 5 which also functions as a horn transmitting the ultrasonic vibration. A chip 11 to be wire bonded is mounted on substrate 10. Also provided is an encoder E which detects the rotational volume of the shaft 3 and outputs pulse which are cumulatively counted by positioning counter 20. The cumulative value represents the present position of the capillary tool 7 as a pulse volume from the encoder. The present position of the capillary tool 7 is transmitted to an arithmetic unit 21. Also included is a command unit 23 which instructs the ultrasonic oscillating circuit 22 to power the ultrasonic vibrator 9 for ultrasonic vibrating.
The contact detecting circuit 24 detects contact between the ball 6a and the bonding face of the chip by observing the impedance of bonding arm 5 to the vibration. When the ball 6a is off the bonding face of the chip, the impedance of the bonding arm 5 is low since the arm can vibrate freely. However, when the bonding arm 5 contacts the bonding face, the impedance of the bonding arm 5 increases sharply since the vibration of the bonding arm is restrained. In response to the sharply increased impedance, the contact detecting circuit 24 outputs a contact detecting signal to the command unit 23.
When performing the wire bonding, the command unit 23 provides a positioning command to the arithmetic unit 21 instructing the target position for the capillary tool. The target position is where the ball 6a is to contact the bonding face of the chip. The arithmetic unit 21 provides the difference between the present position and the target position to driving circuit 25 and the driving circuit 25 supplies the driving current to the voice coil motor 8 corresponding to the difference between the positions. The voice coil motor 8 raises or lowers the bonding arm 5 so that the position difference becomes zero. Ball 6a contacts the bonding face of the chip and, as a result, the impedance of the bonding arm 5 changes. In response, a contact detecting signal is output. When the contact detecting signal is received by the command unit 23, the command unit 23 changes the command signal to a torque command from a positioning command. Driving circuit 25 supplies an electric current to the voice coil 8 to generate the torque command.
FIG. 16 shows the timing chart of the conventional wire bonding.
From Time T1 to T2, the capillary tool lowers from its initial position to a search level at a high speed of V1. The location of the search level is 200 .mu.m to 300 .mu.m above the chip surface.
From T2 to T3, the ultrasonic vibrator 9 generates a soft vibration to detect contact. The capillary tool 7 lowers from the search level to the surface of chip at a low speed of V2.
At Time T3, capillary tool 7 reaches the surface of the chip.
From Time T3 to T4, (.DELTA.T), the capillary tool 7 lowers at a speed of V3 which is lower than V2, crushing the ball 6a on the tip of wire.
At Time T4, the contact detecting circuit 24 provides a contact detecting signal.
Then from Time T4 to T5, the command unit 23 supplies a torque command to the driving circuit 25. The driving circuit 25 supplies the electric current to voice coil motor 8 so that the proper bonding force F, the pressure added on the ball when bonding, can be generated. In addition, the ultrasonic vibrator 9 generates a strong vibration for the bonding.
At Time T5, bonding is completed and the capillary tool returns to the initial position.
FIG. 17 shows the ball 6a on the tip of a wire at times T3, T4, and T5 in FIG. 16. At time T3, the capillary tool reaches the bonding face of chip. At time T4, the capillary tool lowers at a speed of V3 by crushing the ball 6a on the tip of wire at which time the voice coil motor 8 is supplied electric current to generate the proper bonding force F. An excessive load, the bonding force F from the voice coil motor 8 plus inertial force derived from the movable portion, however, is applied instantaneously causing ball 6a to be greatly distorted and which may result in defective bonding or damaged to the chip.
Recently, bonding intervals between bonds have become made smaller because of the use of higher density integrated circuits. As a result, distortion of ball 6a may cause a short with an adjacent bonding point. In addition, chips fabricated from GaAs which are widely used, may be more likely to be damaged because GaAs has weak mechanical strength. Thus, excessive force may cause damage to the chip. Accordingly, excessive bonding load is undesirable. On the other hand, shortening the cycle time to complete the bonding steps is desirable.